Johan Varekamp, Wesleyan University. http://www.wesleyan.edu/ees/JCV/Lobsters%20Report%20final.pdf
Delta 15N (δ15N) is a nitrogen isotope whose abundance in benthic sediment is used to determine the contribution of wastewater and sewage to total nitrogen. Anthropogenic nitrogen loadings from wastewater have higher δ15N levels relative to natural sources. Scientists can use sediment cores to look back in time at changes in δ15N levels and make inferences about historical conditions in Long Island Sound.
δ15N levels indicate the concentration of nitrogen loading in a sediment sample. Higher δ15N levels indicate a greater concentration of nitrogen loading than what would be found in natural sources, thus indicating the presence of high amounts of sewage-related nitrogen from wastewater treatment plant discharges.
The chart above depicts δ15N values over the last 1000 years in Long Island Sound. The increase in sediment δ15N values around 1800 reflects an increase in sewage and wastewater treatment plant discharges into Long Island Sound.
Some of our indicators are categorized as either historical or legacy indicators. The Sediment Delta 15 indicator is a historical indicator. This type of indicator provides perspective on past environmental conditions in Long Island. It is not measured periodically like our other indicators. Many indicators of this type use sediment core samples to measure how conditions in the Sound have changed over hundreds to thousands of years.
A legacy indicator is another type of indicator that is no longer actively tracked by the Long Island Sound Study. This is often due to lack of funding or termination of field sampling programs. A legacy indicator might also be replaced by a more effective indicator or may not need to be updated because the goal has been met. We continue to present these legacy indicators because they can still provide baseline information on conditions in the Sound if the sampling programs resume.